In the production of electrical circuit assemblies it is desirable to effect tests of the conductive pathways and circuit components. A common approach is to apply a multicontact test probe or adapter to the printed circuit board and then evaluate the pathways or components using logic circuitry or other conventional test circuitry.
In general test adapters are used which include a number of individual spring-loaded contact pins. Each contact pin has a casing in which a piston or plunger is accommodated in a movable manner.
The plunger is under the influence of a spring which exerts sufficient force on the plunger so that the latter properly contacts the test sample.
These test adapters are thus usable for simultaneously testing a number of contacts of a corresponding test sample so as to determine whether any, all or no conductors or other electrical or electronic components are intact and meet the intended functions.
Such test samples consist frequently of at least one straight row of contacts which are closely arranged next to one another. Since these contacts must simultaneously be tested, it is required to have the contact pins as closely arranged to one another as possible.
In known spring-loaded contact pins, the casings as well as the plungers have circular cross sections. For loading the plungers within the casing, prestressed compression springs are used which must exert relatively high forces--usually 50-300 cN (grams)--to provide a secure electrical connection of the test contacts with the tips or heads of the plungers.
This requirement limits, however, the possibility of reducing the diameter of the casing of such contact pins.
It has thus been proposed to connect the plunger of each contact pin with a thin wire-like extension which is guided within a hollow needle and in this manner is closely approached to the test sample (see Yearbook of the German Society of Chronometry, vol. 30, 1979, pages 269-276).
Although this technique allows the free ends of the extensions to be squeezed together more closely than is feasible with the contact pins directly, this construction is very complicated to produce, sensitive and rather expensive.
In addition this arrangement has some drawbacks as to function. The long extensions cannot be guided in a frictionless manner within the hollow needles so that malfunctions have been experienced. In particular, the force exerted by the spring on the plunger cannot completely be transmitted onto the extensions. This means that the tip of the plunger may not be provided with sufficient load to achieve an electric connection with the contacts, thus leading to inaccurate results.